Very similar to asking why you should use React when Angular came first. A-Frame and React VR represent different styles, and they both excel at different scenarios.
If you're building a relatively static scene, or really like the idea of having a small amount of HTML with super powers, then run with A-Frame. I think it's a fantastic product.
If you know React, find React's declarative style of programming desirable, or have a more complex, stateful application, then React VR is your best bet. It's true that the A-Frame community is larger than the React VR community, and there's more code out there for it, but React VR integrates with the entire history of React tooling and libraries – React VR doesn't need a specific data management or navigation library, because it's just React. Our goal is to take all web developers who know React and enable them to become VR developers as well.
If you know React, find React's declarative style of programming desirable, or have a more complex, stateful application, then React VR is your best bet. It's true that the A-Frame community is larger than the React VR community, and there's more code out there for it, but React VR integrates with the entire history of React tooling and libraries – React VR doesn't need a specific data management or navigation library, because it's just React. Our goal is to take all web developers who know React and enable them to become VR developers as well.